Virtually all building codes require that drain pipes from sinks, washing machines, and like user stations have water traps, the purpose of which is to prevent unsanitary and unpleasant sewer gases from entering buildings via the drainpipes. These traps work by trapping within a section of drainpipe an amount of water that provides a liquid seal separating sink and sewer. Because this barrier can evaporate or freeze, particularly if a user station is rarely used, such traps are conventionally accompanied by primers that bleed off water from, typically, a cold water supply line to replenish the water barrier. Such a scheme inherently wastes water, and does nothinng to prevent the water seal from freezing because these traps are fed with cold water. In order to minimize waste, prior primers typically have complex valving schemes, for example valves that open upon flow beginning in the feed line, but close upon flow exceeding a predetermined value. These primers are thus complicated devices given to periodic failure, and require periodic maintenance or replacement.